A typical semiconductor device is structured such that an insulating substrate on which a semiconductor element is mounted is disposed on a radiation plate (e.g., Patent Documents 1 and 2). Further, when the radiation plate is soldered to the insulating substrate, a pattern of a solder resist can be previously formed on a surface of the radiation plate so that solder melted in a reflowing step is held in a prescribed position. Typical examples of a material of the solder resist include a phenolic resin and an epoxy resin.